Crosslinked cellulose products are widely known in the art. It is well-known that properties of cellulosic materials can be improved as to wet strength and other properties by crosslinking the cellulose fibers with intrafiber chemical crosslink bonds so that chemical bonds exist between the hydroxyls of the cellulose, thereby replacing weak hydrogen bonds with strong covalent bonds to tie together hydroxyl groups. For example, monomeric amine resins such as methylolated derivatives of urea or melamine react with hydroxyls of the cellulose to replace weak hydrogen bonds with strong covalent bonds. However, such crosslinking agents, known in the art as formaldehyde and formaldehyde addition products, have been known to cause problems. When formaldehyde is used as the sole crosslinking agent, as indicated in U.S. Pat. No. 3,960,483, the process for treating cellulosic fiber containing products such as cloth made of cotton has tended to lack reproducibility because control of the formaldehyde crosslinking reaction has been difficult. Additionally, as indicated in EP Publication 0 427 317 A2, Herron, et al., the commercial appeal of crosslinked fibers with formaldehyde and formaldehyde addition products has suffered due to safety concerns. These crosslinking agents are said to cause irritation to the human skin and have been associated with other human safety concerns such as the irritating effect of formaldehyde vapor on the eyes.
Crosslinked cellulosic bonding agents serve to give both chemical and physical bonding between adjacent fibers. The crosslinked fibers are normally stiffer than their untreated counterparts. As taught in U.S. Pat. No. 5,225,047, the crosslinking can result in considerable fiber embrittlement. A refiberization of a sheeted crosslinked product at point of use can result in an excessive amount of fines in the preparation of fluff products with greater bulk which tend to hold retained liquid under compressive forces encountered during use of the product such as a diaper. U.S. Pat. No. 5,225,047 also teaches use of a debonding agent with a latent cellulose crosslinking agent wherein the crosslinking agent is a polycarboxylic acid or methylolated urea compound to prepare a cellulose product which can be repulped in water without significant fiber breakage.
A particular advantage of this invention is found in the lack of any need to include a debonding agent to depulp the crosslinked cellulosic product.
Methods of improving a cellulosic product by crosslinking, as indicated in U.S. Pat. No. 5,225,047, have used polycarboxylic acids in aqueous compositions as crosslinking agents to obtain a formaldehyde-free cellulose strengthening aqueous composition. Examples of such use of polycarboxylic acids are taught in U.S. Pat. No. 5,427,587. The polyacid must be sufficiently nonvolatile so that it will substantially remain in the treated substrate during heating and curing operations and bear at least two carboxylic acid groups. Suitable examples are citric acid, butane tetracarboxylic acid (BTCA) and cyclobutane tetracarboxylic acid or a polymeric polyacid such as polymethacrylic acid.
However, as noted above in U.S. Pat. No. 5,225,047, use of a crosslinking agent such as a polyacid can result in embrittlement of the cellulosic fibers.
The present invention is applicable to fibrous cellulosic material in the form of textile fibers and woven and non-woven textiles such as yarns and woven or knit fabrics, fibers, linters, roving, slivers, paper, pulp fluff, and wood oriented strand board termed consolidated wood products, the fibrous cellulosic material characterized as containing at least 30 wt. % cellulosic fibers.
The integrity of fibrous cellulosic materials, as noted above, is dependent on the hydrogen bonds which form between the structures of the cellulose fibers. The bonds between hydroxyls of neighboring fibers are very strong in a dry state but the hydrophilic nature of the cellulose and the bonding of water with the cellulose can affect the hydrogen bonds formed between the structures of the cellulose fibers. Fibrous cellulosic materials lose much of their strength when wet.
Improvement in the wet strength of fibrous cellulosic materials by reaction with the hydroxyl groups of the cellulose improves the range of utility of fibrous cellulosic materials. Woven and non-woven textiles are useful in personal hygiene products such as diapers and incontinent products which utilize the absorbency and comfort of fibrous cellulosic materials. Improvement in wet strength of absorbent cellulosic materials improves fluid retention of diapers and incontinent products. Improvement in wet strength of paper helps preserve its stiffness, bursting, tensile and tearing strength when paper is exposed to high humidity or it is wet. Wood oriented strand board, also called consolidated wood products, can exhibit swelling when exposed to water and, in part, can disintegrate under extreme conditions of heat and water exposure.
Construction of incontinent products and diapers is illustrative of applications of improved wet strength fibrous cellulosic materials. Incontinent products and disposable diapers are constructed in layers to maximize comfort and performance. Typical construction, starting at the skin side of the article, consists of a topsheet made from hydrophobic polypropylene fiber, a wicking layer made from cellulosic pulp, an absorbent core consisting of super absorbent polymer particles imbedded in cellulosic pulp, and the backsheet, usually made from polyethylene. The absorbent fiber most commonly used in incontinent products and diapers is cellulosic fiber such as comminuted wood pulp, commonly known as "pulp fluff", or simply "fluff". This invention has utility in crosslinking the pulp fluff used in the wicking layer, which requires good wettability, and proper pore size and pore size distribution. Polycarboxylic acids such as citric acid have been used to crosslink pulp fiber. This invention teaches a more effective crosslinking technology for cellulosic pulp. This modifies the characteristics of the fibrous web. The polyanhydride crosslinking imparts a three-dimensional character to each fiber. This gives the assembly more bulk or lower density with resulting greater fluid distribution and absorption capacity.
The polycarboxylic reaction products of TMA and a diol of the present invention, including, but not limited to hydrolyzed EGBAT resin may also be used to prepare individualized crosslinked fibers. Process for making individualized crosslinked fibers and structures incorporating such fibers are described in U.S. Pat. No. 5,137,537; U.S. Pat. No. 5,190,563; and U.S. Pat. No. 5,183,707 each of which is incorporated herein by reference in its entirety.
Superabsorbent polymer (SAP), in powder or fiber form, can also be incorporated into the absorbent cellulose product. SAP comprises polymers which can form gels containing at least 10 grams of water per gram of polymer. Certain applications require the fiber to be reinforced with long fibers which increase the tear strength. Such long fibers include, but are not limited to, viscose fibers and polyester fibers. The use of such long fibers enables one to form webs which are sufficiently strong to be used in processes in which webs of special density can be cut to size and placed at the desired site in absorbent products. Increased strength is also desirable for products having low grammages in order to ensure that the absorption body does not rupture in the final product.
Products according to the invention having density gradients can be produced by forming and pressing such webs to obtain webs of different densities before being laid together or by pulp having different compressibilities, produced by varying the crosslinking composition with which the cellulose fibers are impregnated. The number of layers used and the densities of the layers will depend on the use of the product. For products, such as diapers and incontinent pads, which are to be loaded with large amounts of liquid over a short time period, it may be desirable to have a first upper layer of low density which faces the wearer of the absorbent product and one or more additional layers beneath and, if desired, connected to the first layer which may be compressed to higher densities. For example, a diaper, may be packed at an average density of about 130 to about 170 kg/m.sup.3. Use of the cellulose pulp of the present invention which has been impregnated with hydrated EGBAT resin provides the possibility of increasing the average density of the absorption body in absorbent products and thus lowering the costs of storing and transporting such products.
The impregnated pulp of the present invention can also be formed into fiber webs of high density using a dry or a wet forming technique, with the fiber being dispersed in air or water as the web is formed on a wire. The webs can be reeled into high density reels which may provide a considerable cost savings in transportation and storing the impregnated pulp as a semi-finished product. The fiber web can later be fibered, for example in a hammer mill, and then formed into products of the desired density or the fiber web can be cut to a desired size for placing in the desired position in an absorbent product such as a diaper, incontinence pad, sanitary napkin, etc. An advantage of the present invention is that pulp containing the impregnated cellulose fibers of the invention which are crosslinked in the dry state can be used in current equipment for producing diapers, incontinence pads, sanitary napkins, and air-formed paper.
It is an object of this invention to provide fibrous cellulosic materials containing at least 30 wt. % cellulosic fibers with improved wet strength by the preparation of an impregnated cellulosic material containing ester bonds between the hydroxyl groups of the cellulose and the anhydride groups of a polyanhydride
It is an object of this invention to provide a crosslinked cellulosic product which can be readily repulped in water to a free fiber condition without excessive fiber breakage or energy input.
It is a further object of this invention to provide a cellulosic product such as paper with improved wet strength, but which is readily repulpable.
It is a further object of this invention to provide a cellulosic product such as pulp fluff with improved wet resilience.
It is a further object of this invention to provide a cellulosic product such as textile fabric with improved durable press characteristics.
It is a further object of this invention to provide wood products such as oriented strand board, also called consolidated wood products, which exhibit less swelling when exposed to water.
It is a further object of this invention to provide an ethylene glycol bis-(anhydrotrimellitate) resin composition and a hydrated ethylene glycol bis-(anhydrotrimellitate) resin composition and aqueous solutions thereof. Such compositions and aqueous solutions are useful in crosslinking cellulose fibers.